Guralp Systems CMG-3ESPCD - Manual (Page 86)

CMG-3ESPCD Digital Broadband Seismometer Inside the 3ESPCD
second, triggered by timing signals from the ARM processor. The DSP can
control all 5 ADCs and process the data in real time.
An important feature of the system design is its ability to synchronise the
sampling of the analogue to digital converter to an external time reference.
This way, data samples are accurately time stamped at source. To keep
sampling accurately in step with UTC, you can synchronise the
microprocessor's time-base to an external reference such as GPS or, in larger
arrays, to a centrally-transmitted time reference. Transmitting a time
reference avoids the cost and power consumption of multiple GPS receivers,
and since it only involves sending 2 characters per second it can utilise a low
band-width, even half-duplex link.
To achieve the high degree of timing precision required for a 24-bit digitiser
system, the microprocessor time-base is run from a precision voltage
controlled oscillator. On-board software keeps this oscillator tuned to the
external reference so that its frequency is accurately set and maintained
through changes in temperature or ageing. Once the system has stabilised,
the control is sufficiently accurate to maintain precision sampling for several
days without an external reference. The system also automatically
compensates for the pure time delay introduced by the digital
filtering/decimation processes in the DSP.
The DSP software consists of 6 cascaded programmable filter/decimation
stages, which allow you to select multiple data output rates simultaneously.
Each stage can be set individually for decimation factors of 2, 4, or 5. Data
can be output at any or all of these rates according to constraints of storage
and transmission bandwidth. For example, a system can be configured to
provide data at 200, 50, and 10 samples/sec, covering the whole of the
seismological broad band range. The configuration of the DSP is
programmable in the field via the host ARM microprocessor.
The primary digital interface for the system is the serial port cards. One serial
port is configured to send the data packets to a local EAM unit for storage, or
via a modem or radio link to a central recording station. A second serial port
is normally used with a local GPS receiver for time synchronization, or
alternatively the first (data) port can be used for time synchronization from a
central station (stream-sync mode).
Each of the serial ports on the digitiser can be configured for a wide range of
standard baud rates (with different settings available for transmit and receive
channels), allowing a wide range of data links to be used depending on the
required data rates. The UARTs and serial port module are optically isolated
to avoid any ground loops that could degrade the performance of the ADCs.
The serial port module also includes 32k of RAM for data buffering and
formatting by the transmission/reception process.
MAN-C3E-0002 86 Issue B - November 2013